- Plesk for Windows
Going to Tools & Settings > Mail Server Settings > Statistics > Spam Statistics > Recipients, there are email addresses that are not existing in Plesk, these email addresses are configured as forwarding emails.
This is a Plesk bug with ID #PPPM-11063 which is planned to be fixed in future Plesk updates.
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This issue doesn't have an impact on Plesk functionality and it can be safely ignored.